1. Field of the Invention
The present invention relates to the formation of a refractory repair mass, and in particular to the formation of a refractory repair mass on the surface of an electrocast refractory material.
2. Description of the Related Art
Electrocast refractory materials are oxides of certain controlled compositions obtained by fusion at very high temperatures, usually in an electric furnace, and by casting the so-produced molten material into moulds. There are several families of electrocast refractory materials, including zirconia-containing materials such as alumina/zirconia/silica (AZS), of which one brand is available under the trade name Zac, alumina/zirconia/silica/chromium (AZSC) and spinels such as magnesia/alumina and chromic oxide/alumina.
Electrocast refractory materials find use in a number of specialist high temperature applications, for example as refractory blocks for those parts of furnaces which are subjected to severe high temperature operational conditions. These conditions are encountered at various points in the superstructure of a glass melting tank, with especially severe conditions being encountered at the xe2x80x9cglass linexe2x80x9d (also known as the xe2x80x9cflux linexe2x80x9d), that is to say at the upper surface of the molten glass.
In the vicinity of the glass line the refractory material of the tank is subjected to direct thermal contact with the hottest layer of liquid glass and immediately above that to thermal contact with the adjacent furnace atmosphere. The liquid and the adjacent gas thus each subject the glass line refractories to substantial but different stresses. As the level of the glass line rises and falls in the course of the production process the refractories in its vicinity undergo significant thermal cycling. In addition to variable thermal stresses occasioned by this cycling, mechanical stresses are imposed by the liquid glass flowing through the tank having a mechanical scouring action.
Despite the high quality of electrocast refractory materials and their excellent suitability for such duties they nevertheless undergo considerable erosion in use. There is accordingly an ongoing need for repairs of the said materials and a requirement for the formed repair itself to be resistant to the severe conditions. Long-term durability of the repair is especially important given that the furnace may be required to operate continuously for a period in excess of ten years.
The present invention is concerned with ceramic welding repairs. xe2x80x9cCeramic weldingxe2x80x9d is the term that has come to be used for a refractory welding procedure first claimed in our GB patent specification 1330894, in which a mixture of refractory oxide particles and combustible particles is projected in an oxygen-containing gas stream against the surface of a substrate material. The combustible particles, typically finely divided silicon and/or aluminium, serve as fuel for combustion with the oxygen, reacting against the target surface in a highly exothermic manner and releasing sufficient heat of combustion to form a coherent refractory mass. There have been many subsequent patent specifications on ceramic welding, including our later cases GB 2110200 and GB 2170191.
Ceramic welding can be employed for making discrete refractory blocks or for binding refractory pieces together but has mostly been employed for the in situ repair of worn or damaged refractory walls of furnaces such as coke ovens, glass furnaces and metallurgical furnaces. Ceramic welding is particularly well suited to the repair of a hot substrate surface, making repairs possible while the equipment remains substantially at its working temperature and if necessary while the furnace as a whole remains in operation.
It is a well-established practice in ceramic welding that the composition of the ceramic welding mixture be chosen to produce a refractory repair mass which has a chemical composition compatible with and preferably similar to that of the furnace constructional material. It has however been found that merely matching the chemical compositions of the refractory substrate material and the repair, mass may not be sufficient to ensure a durable repair. Even with chemical compatibility there can still be a problem in ensuring a strong and lasting bond between the repair mass and the worn or damaged refractory substrate. The problem tends to increase if the repaired surface is subjected to very high temperatures or to thermal cycling.
Thus attention must also be given to the physical compatibility of the repair mass and the refractory substrate, most particularly with regard to their respective degrees of thermal expansion, which is linked to their crystallinity. In our copending application GB-A-2257136, which relates to the repair of a surface based on a silicon compound, steps are taken to produce in the repair mass during its formation a crystalline lattice which resembles that of the base refractory material, with a view to avoiding the problem of the formed mass becoming separated and detached from the base refractory material. With such silicon-based repair surfaces it is especially important to avoid the formation of a vitreous phase in the repair mass.
Surprisingly it has now been found that in the case of electrocast materials a feature which is necessary to ensure physical compatibility between the repair surface and the, repair mass is the presence of a vitreous phase. As a result it has been found that high quality durable repairs can be effected at such hostile locations as the glass line in a glass melting tank by ensuring the presence of a vitreous phase in the repair mass.
Thus according to the present invention there is provided a process for the formation of a coherent refractory repair mass on a surface of electrocast refractory material, in which process a powder mixture of combustible particles and refractory particles is projected in an oxygen-containing gas stream against the refractory surface and the combustible particles react against the said surface in a highly exothermic manner with the projected oxygen and thereby release sufficient heat of combustion to form the repair mass, characterised in that the powder mixture includes at least one constituent which enhances the production of a vitreous phase in the repair mass.
The invention also provides a powder mixture for the formation of a coherent refractory repair mass on a surface of electrocast refractory material, which mixture includes combustible particles and refractory particles for projection in an oxygen-containing gas stream against the refractory surface, where the combustible particles react against the said surface in a highly exothermic manner with the projected oxygen and thereby release sufficient heat of combustion to form the repair mass, characterised in that the powder mixture includes at least one constituent which enhances the production of a vitreous phase in the repair mass.
The invention is especially well suited to the repair of an electrocast zirconiferous refractory material, employing a powder mixture which comprises zirconia-containing refractory particles.
According to the invention the powder mixture containing a constituent which enhances the production of a vitreous phase in the repair mass is applied directly to the surface of the electrocast refractory material to be repaired. The presence of a vitreous phase in the repair mass has been found to provide the benefits of improving both the adhesion and maintenance of adhesion of the repair mass to the electrocast refractory surface. The vitreous phase exists in the bonding phase of the mass and resembles the vitreous phase which exists in the refractory material beneath the surface.
A particular advantage is that the vitreous phase expands and contracts in the same way in both the repair mass and the substrate. Moreover in the case of zirconiferous electrocast refractory material the vitreous phase absorbs both the contraction of zirconia (ZrO2) which occurs with the allotropic transformation from the monoclinic form to the quadratic form at around 1100xc2x0 C., and the expansion which occurs in the reverse direction.
The presence of the vitreous phase reduces the porosity of the repair mass and together with the good dispersion of the zirconia therein enhances its corrosion resistance.
The improved repair masses of the invention thus provide increased reliability of repairs to furnace superstructures formed of electrocast materials. They are of particular interest for the repair of glass furnaces reaching the end of a campaign and for which a conventional repair using a sacrificial protection of electrocast refractories is not possible.
The combustible particles, which serve as the fuel in the powder mixture, are preferably selected from silicon and aluminium. Their average particle size should be less than 50 xcexcm and is preferably in the range 5 to 15 xcexcm. The term xe2x80x9caverage particle sizexe2x80x9d is used herein to refer to the particle diameter above which 50% by weight of the particles have a greater diameter and below which 50% by weight have a smaller diameter. The total amount of combustible particles in the powder mixture is preferably in the range 8 to 15% by weight.
The total amount of refractory particles in the powder mixture is preferably at least 70% by weight, most preferably at least 75% by weight. Such high proportions assist in ensuring the production of a homogeneous repair mass. The proportion of any zirconia in the powder mixture should be at least 25% by weight, preferably at least 40% by weight, so as to assist in ensuring the heat-resistant properties of the repair mass. In addition to containing zirconia the powder mixture may contain other refractory materials, for example alumina or silica.
Convenient sources of the refractory particles are provided by alumina-zirconia eutectic alloys. The eutectic alloy is readily produced by electromelting. A preferred chemical composition of the alloy is given by the eutectic composition of approximately 55% Al2O3 and approximately 40% ZrO2. Such an alloy is suitable for the repair of refractories in the family of AZS 41 electrocasts (approximate ZrO2 content 41% by weight), which are especially resistant to corrosion by sodium/calcium glass.
If desired the abovementioned eutectic alloys can be used in combination with additional quantities of the refractory materials such as alumina, zirconia and silica.
The average particle size of the refractory oxides such as zirconia and alumina, if employed as discrete particles, is preferably in the range 100 to 200 xcexcm. The maximum particle size of any silica employed as discrete particles in the powder mixture is preferably in the range 1.0 to 2.5 mm. In the case of the eutectic alloys the maximum particle size is preferably in the range 0.8 to 1.2 mm.
The use of a eutectic alloy of the type mentioned above permits the achievement of AZS masses with an improved dispersion of zirconia and alumina throughout the mass. It has also been observed that nodules of zirconia are found at the grain boundaries of the eutectic material. The benefits of using the eutectic alloy are therefore:
in the case of thermal variations, the improved dispersion prevents localised stresses in the material. In masses where the alumina (as corundum, Al2O3) and the zirconia were introduced separately these stresses may create micro-fissures around the particles of corundum;
in the case of contact with molten glass, the nodules of zirconia provide protection for the whole eutectic particle, there being no corundum particles in the weld which are not protected by zirconia.
The powder constituent which enhances the production of a vitreous phase in the repair mass is added in particulate form, preferably having an average particle size in the range of 100 to 500 xcexcm. Preferred examples of the said constituent, also referred to herein as the xe2x80x9cvitrifying agentxe2x80x9d, are sodium carbonate, sodium sulphate, sodium oxide, potassium carbonate, potassium sulphate and potassium oxide. In general the said constituent is preferably present in the vitreous phase as the oxide, which can be achieved either by adding it to the powder mixture as the oxide or by adding it as a salt which generates the oxide under the exothermic conditions at the repair surface.
The amount of the abovementioned vitrifying agents is preferably in the range 2 to 10% by weight of the powder mixture.
The vitreous phase as such in the repair mass is usually a silicate phase, although it can alternatively be formed by one or more oxides of boron or phosphorus.
When the combustible particles include silicon particles the product of the exothermic reaction with oxygen includes silica which is incorporated in the repair mass and can assist in the formation of a vitreous phase therein.
An alternative or additional vitrifying agent is provided by silica employed in an amount in excess of the amount notionally required as a refractory constituent of the powder mixture. The maximum particle size of the silica is preferably in the range 1.0 to 2.5 mm.
In one embodiment of the invention the formation of a repair mass from the powder mixture (the xe2x80x9cfirst powder mixturexe2x80x9d) including at least one constituent which enhances the production of a vitreous phase therein is followed by the formation on the said repair mass of a further coherent refractory repair mass. In this embodiment the repair mass applied according to the invention serves as a base coating on the material to be repaired. The further coherent refractory mass thus forms a further coating, creating a sandwich of repair layers on the electrocast refractory surface. The base layer contains a vitreous phase but the second layer contains little or no vitreous phase. The powder mixture used to form the further coating should contain little or no vitrifying agent